Travel 50 kilometers north and you will arrive in what by comparison is the urban sprawl of the Monterey Peninsula. Once described by John Steinbeck as “a poem, a stink, a grating noise,” the former industrial fishing city of Monterey, now mostly a tourist town, spills into the ocean. Beach houses, looming over the waves, are densely packed along kilometers of the coastline. Meanwhile, the gigantic 400-hectare farms of Salinas lie adjacent to the north, meaning fertilizer runoff and sewage flow daily into the Monterey Bay.

Such were the backdrops for one of the most expansive studies on sea otters ever conducted. Commissioned by the California Coastal Conservancy (CCC) and the U.S. Fish and Wildlife Service (FWS), it explored the fabled question: City versus country, which is better? And the winner was surprising. “The main things we found were neither intuitive or what we expected to find,” says biologist Tim Tinker of the U.S. Geological Survey (USGS), lead author on the 250-page study accepted by the CCC and FWS in January.

Despite Monterey’s environmental sins, city otters are catching fewer diseases, finding food more easily and having more success at reproducing. To learn this, a team of 25 marine biologists and veterinarians became a quasi-National Security Agency for otters. Employing an arsenal of surveillance tools they monitored 140 otters, split evenly between Big Sur and Monterey, day and night from 2008 to 2012.

The otters were retrofitted with radio biosensors that transmitted their locations and measured indicators such as bodily temperature or how deep the playful marine mammals would dive to collect food. “Every time we add a new technique it opens new doorways and raises new questions,” Tinker says. “We can now see if they are fighting an infection up to four weeks before they die. Females have a distinct temperature signal when they’re in estrous, then again when they're closer to pregnancy, and when they give birth—so now we have a much better understanding of reproduction in this species.”

When one of the 140 otters became sick or died, the team moved in quickly to identify the potential cause. They turned to genetic tests developed during Alaska’s Exxon Valdez spill, where otters and other marine mammals were exposed to a surfeit of chemicals and carcinogens. “You'd expect animals around a more urbanized estuary, such as Monterey Bay, to have more exposure to oil products from cars or disease elements relative to Big Sur,” says co-author Keith Miles, a geneticist with the USGS Western Ecological Research Center.

City wastewater is packed with land-borne pathogens, such as Toxoplasma gondii, the cat-loving, mind-altering parasite that pregnant women try to avoid. The diseases are flushed out to the sea in kitty litter and other sewage, harming marine mammals such as dolphins, beluga whales, monk seals and sea otters.

Sea otters survive exclusively on seafloor invertebrates, like crabs and mussels, which are essentially water filters prone to taking up pollution and infectious diseases. And yet, the “country” otters were worse off than those near Monterey. They weighed less, a sign of a poorer constitution, and scored worse on physical exams. They were more likely to show signs T. gondii infection and had higher cholesterol, although it is unclear if the latter has the same consequences in otters as it does in humans. Their biggest struggle, however, was with their search for sustenance. “You can pretty much explain 90 percent of the variation in survival and reproduction on the per capita abundance of food,” Tinker says.

During the daytime, biologists with binoculars stood watch on nearby beaches or in kayaks, waiting for the furry mammals to surface with food. Floating on their backs in the sunshine with stomachs as lunch tables, the otters feasted while scientists recorded what they ate. The researchers tracked twilight snacking by examining the mammals’ whiskers: When otters eat, the nutrients enter their bloodstreams and ultimately flow into the cells that produce whiskers. A chemical signature of isotopes builds in the hairs as they grow, creating a time line of the otters’ meals over the last year. The pantry was better stocked in the waters off Monterey, and with otters being picky eaters, the menu matters. “At the population level, otters are generalists that consume almost every type of benthic invertebrate there is, but at the individual level, they are dietary specialists,” Tinker says.

Unlike other marine mammals, otters do not produce blubber, and their extremely high metabolisms require consistent and continual feeding to produce enough warmth to live in the chilly waters of northern California. Otters consume a quarter of their body weight every day, making their diets especially crucial to survival.

Prey diversity and abundance were marginally better in an urban environment. If a city otter could not find its first choice of prey, usually abalone, it could fall back on its second, such as crabs or urchins. Sea urchins in particular satisfy a special niche in the otters’ diet as a high-energy food source, and there were fewer to go around at Big Sur.

It’s like when the one sushi place closes in a small town, there is nowhere else to go, but in a big city there might be a sushi restaurant on every block. Ultimately, this meant country otters had to work harder to obtain the same number of calories.

Snails, a frequent carrier of T. gondii, were more prevalent in the diets of the Big Sur otters, potentially explaining why the disease was more common among country otters. Otters who preferred snails as their primary food choice were 42 times more likely to be infected with this pathogen.

Female otters and their pups near Big Sur suffered the most from the dearth of food diversity, spotlighting a marquee result from the study. “Sea otters, demographically speaking, are really, really local,” he says. “Unlike harbor seals, elephant seals and other marine mammals that range over hundreds or even thousand of miles, sea otters stick to their roots.”

For years, scientists have posed questions about "the California population," as if it were a homogenous entity, but now they realize that otters, especially adult females, rarely leave their original 10- to 30-kilometer stretch of coastline. The Big Sur otters thus represent a demographically distinct “community” from their neighbors just 50 kilometers away at Monterey, with different food conditions and facing different threats. Tinker and his colleagues will present these findings at a February meeting of California sea otter biologists at the Seymour Marine Discovery Center in Santa Cruz.

Because of the extra demands of child rearing—weaning or foraging for extra mouths—Big Sur females were at a disadvantage. Some developed end-lactation syndrome, a condition in which they couldn’t find enough food to sustain themselves and their pups, leading to malnourishment and often death for both. “Otters have to be supreme athletes to satisfy their hunting demands. Pregnancy and pup care are hugely expensive,” says co-author Melissa Miller, a wildlife pathologist at the California Department of Fish and Wildlife (CDFW). She describes the syndrome as an Achilles' heel for California otters, whose population is starting to rebound from nearly going extinct in the 1980s.

Beyond identifying the relative suffering of Big Sur otters, the recent survey also offered insight into a new problem faced by the animals in both the city and country. Sharks are known to be an emerging threat to the recovery of the California otters. In particular, white shark attacks, once relatively rare, now account for half of otter deaths along the central coast, Tinker says.

California’s recent boost in marine fertility, with whales, sea lions and elephant seals appearing in record numbers, is attracting the sharks toward the otters’ coastal realm, Miller says. The sleek predators don’t actually eat the otters. They bite, realize it isn’t a delicious seal or sea lion, and let the injured otter go. But puncturing the otters’ wetsuitlike skin exposes them to the cold as well as lethal infections.

This underwater siege was revealed only by keeping constant watch over the sea otters, which is “the power of this work,” she adds. While much was garnered on how otters live, only a handful of tagged otters passed away during the current study, so there is still more to learn about how they die, she says. Over the past 17 years the USGS and CDFW have amassed one of the largest libraries of otter biology, encompassing data on 6,300 living and stranded animals.